U-shaped heat exchanger tube with a concavity formed into its return bend

ABSTRACT

A heat exchanger tube includes a tube body that forms a hollow passageway and has a U-shaped tube section defining a return bend and a pair of straight tube sections. Respective ones of the straight tube sections are connected to the U-shaped section at respective connection locations and extend generally parallel to one another to define an internal space disposed between and among the U-shaped tube section and the straight tube sections connected to the U-shaped tube section. The U-shaped tube section has a concavity formed thereinto with the concavity defining a portion of the internal space. A heat exchanger serpentine tube includes a plurality of straight tube sections arranged in a plurality of generally parallel rows and disposed in a common plane and a plurality of U-shaped tube sections connected to the plurality of straight tube sections to form a serpentine configuration.

FIELD OF THE INVENTION

The present invention relates to a U-shaped heat exchanger tube. More particularly, the present invention is directed to a U-shaped heat exchanger tube with a concavity formed into a return bend of the U-shaped heat exchanger tube.

BACKGROUND OF THE INVENTION

Commonly known in the art in the fabrication of the heat exchangers, straight tubes are bent in an approximate range of bend angles of 170° and 190° so as to form a unitary construction of two straight tube sections integrally connected with a return bend formed at a selected bend angle. A skilled artisan would appreciate that if the straight tube is bent precisely 180°, the two straight tube sections would extend parallel to one another while if the straight tube is bent at a selected bend angle anywhere in the approximate range other than 180°, the straight tube sections would extend generally parallel with one another. For simplicity, the term “generally parallel” shall refer to the relationship of the two straight tube sections after the straight tube is bent at any selected angle in the approximate range of 170° and 190° including the precise bend angle of 180°.

Depending upon the metal material used for the fabrication of the heat exchanger tubes, difficulty may arise. For instance, it is difficult to bend straight stainless steel tubes into U-shaped heat exchanger tubes particularly when a relatively tight return bend is desired. Bending, for example, a straight stainless steel tube to yield a relatively tight return bend often results in wrinkling of the tube perpendicularly to the inner radius IR of the bend 4 as illustrated in FIG. 1. Such wrinkling is sometimes considered unsightly and sometimes customers who purchase heat exchangers question whether the structural integrity of the stainless steel heat exchanger tubes with relatively tight, wrinkled bends is compromised.

A conventional method for bending tubes for heat exchangers is described in U.S. Pat. No. 5,142,895 to Schuchert. FIGS. 2-6 illustrate sequential steps in the process of forming a tubular heat exchanger from straight stainless steel or aluminized steel tube. FIG. 2 shows a tube bend tooling 6 that includes a bend die 8, clamp die 10, a pressure die 12, a plastic plug mandrel 14 and a plastic follower 16. The bend die 8 is a split die having symmetrical upper and lower sections 8 a and 8 b which, as shown in FIG. 6, can be vertically separated at a midportion. When sections 8 a and 8 b are fitted together, these sections form a generally circular or cylindrical block having a horizontal tube groove 18 that has a generally elliptical curvature and is adapted for receiving a tube 20 or pipe. The tube groove 18 has a plurality of vertical elongated controlled-wrinkle serrations 22 that are disposed in an arc greater than 180 degrees. That is, the serrations 22 extend beyond the tangents of the bend portion of bend die 8. The centerline radius of the bend die is here approximately 2.5 inches. That is, the distance from the rotational axis of bend die 8 to the entrance of tube groove 18 is such that tube bent with the bend die 8 has a centerline radius of approximately 2.5 inches. Grip section 24 has a grip section tube groove 26 conforming to tube groove 18 except that it is linear and extends tangentially from tube groove 18. As is conventional, the bend die 8 is mounted to a rotary drive 28 such that the bend die 8 can be rotated during bending.

The pressure die 12 and the clamp die 10 have respective linear tube grooves 30 and 32 that may preferably be elliptically shaped and adapted for receiving the tube 20. Initially, the pressure die 12 and the clamp die 10 are aligned side-by-side with tube grooves 30 and 32 linearly aligned and are spaced from an axis defined by tube groove 18 and the grip section 24. The plastic follower 16 having an arcuate surface generally conforming to the outer diameter of the tube 20 being bent is mounted behind the bend die 8 opposite the pressure die 12. A mandrel rod 34 with the plastic plug mandrel 14 on the end extends forwardly with the bend die 8 and the plastic follower 16 on one side and the pressure die 12 and the clamp die 10 on the opposite side. The drive mechanisms for the bend die 8, the pressure die 12, the clamp die 10, the mandrel rod 34, and the plastic follower 16 are not described in detail because they are conventional and an explanation of them is not necessary for understanding the invention.

Referring to FIG. 3, the tube 20 is positioned on the mandrel rod 34 and is held in place by a collet 36. The pressure die 12 and the clamp die 10 are then moved laterally so as to engage the tube 20. More specifically, the clamp die 10 is moved diametrically to the grip section 24 such that the face edges 38 of clamp die 10 respectively seat in conforming grip section notches 40 that are adjacent the grip section tube groove 26. Accordingly, the clamp die 10 and the grip section 24 are interlocked and the tube 20 is firmly clamped therebetween. Similarly, a portion of the tube 20 immediately behind the clamp die 10 is received in the linear tube groove 30 of the pressure die 12. Lateral pressure exerted on the tube 20 by the pressure die 12 is restrained by the plastic follower 16. Also, a portion of face edges 38 (see FIG. 4) of the pressure die 12 seat in and interlock with conforming notches 42 of the bend die 8.

Referring to FIG. 4, the bend die 8 and the clamp die 10 are rotated in unison while the pressure die 12 drives linearly forward with portions of face edges 44 continuously being seated in the conforming notches 42. The tube 20, which remains held by the collet 36, is driven forwardly to the bend point of the bend die 8. The plastic follower 16 has a relatively low coefficient of friction such that the tube 20 readily slides over it while the plastic follower 16 continues to restrain the pressure of the pressure die 12. During the bending process, the tube 20 continues to be clamped between the clamp die 10 and the grip section 24 as the clamp die 10 is driven by a suitable rotating arm 46. As the tube 20 bends around the rotating bend die 8, the inside of the tube bend is compressed and the metal flows into the elongated vertical serrations 22 thereby forming controlled wrinkles 48.

Referring to FIG. 5, the tube 20 is shown after it has been bent a full 180 degrees such that segments 20 a and 20 b are parallel. In such a state, the bend die 8 has rotated 180 degrees from its initial orientation and likewise the clamp die 10 has been rotated 180 degrees about the central axis of the bend die 8 such that the linear tube groove 32 now faces in the opposite direction from its initial position and still clamps the tube 20 to the grip section 24 of the bend die 8. Also, the pressure die 12 is shown to have linearly traversed to its forwardmost position where it still engages the tube 20 at its tangency point to the bend die 8. During the entire bending process, the plastic plug mandrel 14 remains in a stationary position within the tube 20 and thereby functions to limit the collapse of the pipe 20. More specifically, the plastic plug mandrel 14 does not advance around the bend as a multi-ball mandrel would but rather remains stationary with its tip being in an approximate region of the bend point. The plastic plug mandrel 14 is subject to wear that particularly occurs on the outside as the wall of the pipe 20 slides against it but the plastic plug mandrels 14 are relatively inexpensive to replace. As the plastic wears, the plastic plug mandrel 14 is moved slightly forward by a simple machine adjustment so that the tip remains properly positioned to control collapse to the desired degree. In an alternate embodiment, tubes 20 may be bent without using a plastic plug mandrel or any other internal supporting structure. In other words, the tubes 20 can be bent as shown in FIGS. 2-6 without any collapse suppressing structure on the inside.

Referring to FIG. 6, the pressure die 12 and the clamp die 10 are moved in respective directions away from bend die 8 so as to release the tube 20. Also, the upper section 8 a of the bend die 8 is separated from the lower section 8 b using a suitable apparatus so that the tube 20 can be removed from the bend die 8. More specifically, the flow of metal from the inside bends of the tube 20 into serrations 22 prevents the removal of the tube 20 from the bend die 8 without first splitting the bend die 8 and raising the tube 20 so that the tube 20 can be advanced forward for the next sequential rotation and bend. That is, with a relatively large angle bend such as 180 degrees as described herein and especially with the serrations 22 being disposed in an arc greater than 180 degrees so as to provide control wrinkles beyond the inner tangent points, the tube 20 could not be removed horizontally from bend die 8 because the wrinkles 48 near the bend extremities engaged the corresponding serrations 22. Typically, the upper section 8 a of the bend die 8 may be raised approximately ¾ inches and then the tube 20 raised ⅜ inches to free it. Once the tube 20 is disengaged from the bend die 8, sequential bends may be made to the tube 20 by repeating the same process. That is, the upper section 8 a of the bend die 8 is re-engaged to the lower section 8 b and the bend die 8 is rotated clockwise as shown back to the original orientation as shown in FIG. 2. Also, the clamp die 10 is rotated back adjacent the pressure die 12 and both are moved rearwardly to the starting position as shown in FIG. 2. Then, the tube 20 is moved forwardly to a new bend position and preferably rotated on its axis so that subsequent parallel segments are not linearly disposed with segments 20 a and 20 b. That is, the tube 20 may rotated in opposite directions from bend-to-bend so that the serpentine segments are vertically staggered so as to provide a desirable low profile arrangement for a tubular heat exchanger.

FIG. 1 shows a sectioned view of tube 20 after being bent in accordance with the conventional bending method with the bend of the tube having controlled wrinkles 48. As shown, there are controlled wrinkles 48 on the inside of the bend and some of the wrinkles 48 extend beyond a 180 degree arc. That is, the wrinkles 48 extend-beyond the tangent points that provide the bend arc which makes segments 20 a and b parallel with each other.

It would be beneficial to provide a heat exchanger tube with a tight return bend, particularly one that is fabricated from a material difficult to bend, that is wrinkle free. It would also be advantageous to provide a heat exchanger tube with a tight return bend that results in a reduced pressure loss of the heat exchange fluid as the heat exchange fluid flows therethrough. The present invention provides such benefits and advantages.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is to provide a heat exchanger tube having a U-shaped tube section defining a return bend and a pair of straight tube sections with the U-shaped tube section having a concavity formed thereinto.

It is another object of the invention to provide a heat exchanger tube with a tight return bend that is wrinkle free.

Yet another object to the invention is to provide a heat exchanger tube with a tight return bend with a concavity formed into the tight return bend that results in a reduced pressure loss of the heat exchange fluid as the heat exchange fluid flows therethrough.

It is yet another object of the invention to provide a wrinkle-free heat exchanger tube with a tight return bend and with a concavity formed thereinto that is fabricated from a material difficult to bend.

Accordingly, a heat exchanger tube and a heat exchanger serpentine tube of the present invention are hereinafter described.

According to one exemplary embodiment of the present invention, a heat exchanger tube includes a tube body that forms a hollow passageway and has a U-shaped tube section defining a return bend and a pair of straight tube sections. Respective ones of the straight tube sections are connected to the U-shaped section at respective connection locations and extend generally parallel to one another to define an internal space disposed between and among the U-shaped tube section and the straight tube sections connected to the U-shaped tube section. The U-shaped tube section has a concavity formed thereinto with the concavity defining a portion of the internal space.

According to another exemplary embodiment of the present invention, a heat exchanger serpentine tube includes a plurality of straight tube sections and a plurality of U-shaped tube sections. The plurality of straight tube sections are arranged in a plurality of generally parallel rows and disposed in a common plane. The plurality of U-shaped tube sections are connected to the plurality of straight tube sections in a manner such that a respective one of the U-shaped tube sections defining a return bend connects sequential ones of the plurality of straight tube sections to form a serpentine configuration. Sequential ones of straight tube sections connected to a respective one of the U-shaped sections define an internal space disposed between and among the respective U-shaped tube section and the sequential ones of the straight tube sections. Each one of the U-shaped tube sections has a concavity formed thereinto with the concavity defining a portion of the internal space.

Another exemplary embodiment of the present invention is a heat exchanger that includes an inlet header, an inlet connection connected to the inlet header, an outlet header, an outlet connection connected to the outlet header and a plurality of heat exchanger serpentine tube bodies. The plurality of serpentine tube bodies interconnects the inlet and outlet headers.

These objects and other advantages of the present invention will be better appreciated in view of the detailed description of the exemplary embodiments of the present invention with reference to the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view of a prior art heat exchanger tube having a bend with wrinkles of formed therein.

FIG. 2 is a perspective view of prior art tube bending tooling prior to receiving a heat exchanger tube to be bent.

FIG. 3 is a perspective view of the prior art tube bending tooling with the heat exchanger tube being clamped in the prior art tube bending tooling before bending.

FIG. 4 is a perspective view of the prior art tube bending tooling with the heat exchanger tube bent at approximately 90°.

FIG. 5 is a perspective view of the prior art tube bending tooling with the heat exchanger tube bent at approximately 180°.

FIG. 6 is a perspective view of the prior art tube bending tooling with the heat exchanger tube bent at approximately 180° being removed therefrom.

FIG. 7 is a perspective view of a first exemplary embodiment of a heat exchanger tube of the present invention a U-shaped tube section defining a return bend with a concavity formed into the return bend and a pair of straight tube sections connected to the U-shaped section.

FIG. 8 is a top planar view of the first exemplary embodiment of the heat exchanger tube of the present invention shown in FIG. 7.

FIG. 9 is a cross-sectional view of the first exemplary embodiment of the heat exchanger tube taken along line 9-9 in FIG. 8.

FIG. 10 is a cross-sectional view of the first exemplary embodiment of the heat exchanger tube taken along line 10-10 in FIG. 8.

FIG. 11 is a cross-sectional view of the first exemplary embodiment of the heat exchanger tube taken along line 11-11 in FIG. 8.

FIG. 12 is a perspective view of a second exemplary embodiment of a heat exchanger serpentine tube of the present invention.

FIG. 13 is a perspective view of a third exemplary embodiment of a heat exchanger tube of the present invention having a generally elliptical-shaped cross-section.

FIG. 14 is a top planar view of the third exemplary embodiment of the heat exchanger tube shown in FIG. 13.

FIG. 15 is a cross-sectional view of the third exemplary embodiment of the heat exchanger tube taken along line 15-15 in FIG. 14.

FIG. 16 is a cross-sectional view of the third exemplary embodiment of the heat exchanger tube taken along line 16-16 in FIG. 14.

FIG. 17 is a cross-sectional view of the third exemplary embodiment of the heat exchanger tube taken along line 17-17 in FIG. 14.

FIG. 18 is a top planar view of a fourth exemplary embodiment of the heat exchanger tube of the present invention.

FIG. 19 is a cross-sectional view of the fourth exemplary embodiment of the heat exchanger tube taken along line 19-19 in FIG. 18.

FIG. 20 is a cross-sectional view of the fourth exemplary embodiment of the heat exchanger tube taken along line 20-20 in FIG. 18.

FIG. 21 is a cross-sectional view of the fourth exemplary embodiment of the heat exchanger tube taken along line 21-21 in FIG. 18.

FIG. 22 is a top planar view of the fifth exemplary embodiment of the heat exchanger tube of the present invention.

FIG. 23 is a cross-sectional view of the fifth exemplary embodiment of the heat exchanger tube taken along line 23-23 in FIG. 22.

FIG. 24 is a cross-sectional view of the fifth exemplary embodiment of the heat exchanger tube taken along line 24-24 in FIG. 22.

FIG. 25 is a cross-sectional view of the fifth exemplary embodiment of the heat exchanger tube taken along line 25-25 in FIG. 22.

FIG. 26 is a perspective view of a bend die.

FIG. 27 is a cross-sectional view of the bend die shown in FIG. 26.

FIG. 28 is a perspective view of a sixth exemplary embodiment of a heat exchanger.

FIG. 29 is a cross-sectional view of a generally symmetrical U-shaped tube section of a heat exchanger tube formed with a concavity.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Hereinafter, embodiments of the present invention will be described with reference to the attached drawings. The structural components common to those of the prior art and the structural components common to respective embodiments of the present invention will be represented by the same symbols and repeated description thereof will be omitted.

A first exemplary embodiment of a heat exchanger tube 50 of the present invention is hereinafter described with reference to FIGS. 7-11. However, one of ordinary skill in the art would appreciate that the heat exchanger tube 50 can be any type of tube and is not limited to its use for a heat exchanger. As best shown in FIGS. 7 and 8, the heat exchanger tube 50 includes a tube body 52 that forms a hollow passageway 54. The heat exchanger tube 50 has a U-shaped tube section 56 that defines a return bend and a pair of straight tube sections 58 a and 58 b. Respective ones of the straight tube sections 58 a and 58 b are connected to the U-shaped section 56 at respective connection locations CL1 and CL2 represented by dashed lines. A skilled artisan would appreciate that the connection locations CL1 and CL2 are approximate locations only that represent approximately where the straight tube sections 58 a and 58 b are integrally connected to the U-shaped section 56.

The straight tube sections 58 a and 58 b extend generally parallel to one another to define an internal space IS as best shown in FIG. 7. The internal space IS is disposed between and among the U-shaped tube section 56 and the straight tube sections 58 a and 58 b connected to the U-shaped tube section 56 as well as between the double dashed lines DDL. Further, the U-shaped tube section 56 has a concavity 60 formed into itself.

With reference to FIGS. 7 and 8, the concavity 60 defines a portion of the internal space IS. The concavity 60 extends continuously radially along the U-shaped tube section 56 as represented by the arcuate arrow X. The concavity 60 terminates adjacent the respective ones of the connection locations CL1 and CL2. The concavity 60 has a generally constant depth Dc as the concavity 60 extends continuously radially along the U-shaped tube section 56 as represented by the arcuate arrow Y. Further, the concavity 60 has a generally tapering depth Dt that lessens commencing from where the constant depth Dc ends and as the concavity 60 terminates adjacent the respective connection locations CL1 and CL2.

As shown in FIG. 9, the respective ones of the straight tube sections 58 a and 58 b as viewed in cross-section are circular in cross-section. As shown in FIGS. 10 and 11, the U-shaped tube section 56 as viewed in cross-section is generally kidney-shaped. The concavity 60 is also configured in cross-section generally as a U-shape. With reference to FIGS. 10 and 11, the U-shaped tube section 56 includes a main piece 62 integrally connected to the concavity as viewed in cross-section to define a continuous loop. The concavity 60 forms an apex 64 that projects into the hollow passageway 54. In FIG. 11, the main piece 62 has a first imaginary reference point 66 disposed diametric to the apex 64, a second imaginary reference point 68 and a third imaginary reference point 70. A first axis Af as viewed in cross-section extends through the apex 64 and the first imaginary reference point 66 which, in turn, divides the U-shaped tube section 56 generally symmetrically in imaginary half-sections 56 a and 56 b with the apex 64 and the first imaginary reference point 66 being disposed apart from one another at a distance therebetween to define a height H. A second axis As as viewed in cross-section perpendicularly intersects the first axis Af and extends through the second imaginary reference point 68 disposed on the half-sections 56 b and the third imaginary reference point 70 disposed on the other remaining half-section 56 a. The second and third imaginary reference points 68 and 70 respectively are disposed apart from one another at a widest distance therebetween on the respective half-sections 56 a and 56 b to define a width W. A ratio of the height H to the width W (i.e. H/W) is in a range of approximately 0.6 and 0.9 based upon inside dimensions of the U-shaped tube section 56 as reflected in FIG. 11.

With reference to FIG. 9, one of the straight tube sections 58 a or 58 b defines in cross-section a straight tube cross-sectional area XAst of the hollow passageway 54. With reference to FIG. 11, the U-shaped tube section 56 defines in cross-section a U-shaped tube cross-sectional area XAu of the hollow passageway 54. The U-shaped tube cross-sectional area XAu is smaller than the straight tube cross-sectional area XAst. It is estimated that the U-shaped tube cross-sectional area XAu is smaller than the straight tube cross-sectional area XAst by an amount of approximately 20%. However, it is believed that the U-shaped tube cross-sectional area XAu can be smaller than the straight tube cross-sectional area XAst by an approximate range of 10% and 30%.

It was expected, due to the smaller U-shaped tube cross-sectional area XAu relative to the straight tube cross-sectional area XAst, that the pressure loss of heat exchanger fluid flowing through the U-shaped tube section 56 would rise. However, an unexpected result occurred. As determined empirically, the pressure loss of the heat exchanger fluid flowing through the U-shaped tube section 56 actually reduced.

A second exemplary embodiment of a heat exchanger serpentine tube 80 of the present invention is illustrated in FIG. 12. A skilled artisan would appreciate that the heat exchanger serpentine tube 80 can be any type of a tubular serpentine tube for other uses. The heat exchanger serpentine tube 80 includes a plurality of straight tube sections 56 a, 56 b, . . . 56 n and a plurality of U-shaped tube sections 56. The plurality of straight tube sections 56 a, 56 b, . . . 56 n are arranged in a plurality of generally parallel rows and disposed in a common plane P. The plurality of U-shaped tube sections 56 are connected to the plurality of straight tube sections 56 a, 56 b, . . . 56 n in a manner such that a respective one of the U-shaped tube sections 56 defining a return bend connects sequential ones of the plurality of straight tube sections 56 a, 56 b, . . . 56 n to form a serpentine configuration. Each of the sequential ones of straight tube sections 56 a, 56 b, . . . 56 n are connected to a respective one of the U-shaped sections 56 to define the internal space IS. The internal space IS is disposed between and among the respective U-shaped tube section 56 and the sequential ones of the straight tube sections 56 a and 56 b, for example. Each one of the U-shaped tube sections 56 has the concavity 60 as described above.

Each U-shaped tube section 56 and sequential ones of the straight tube sections 56 a, 56 b, . . . 56 n are integrally connected at respective connection locations as discussed above. Similarly, as discussed above, each concavity 60 extends continuously and radially along each one of the U-shaped tube sections 56 and terminates adjacent the sequential ones of the straight tube sections 56 a and 56 b, for example, at the respective connection locations.

As shown in FIGS. 13-17, a third exemplary embodiment of a heat exchanger tube 350 of the present invention has a generally elliptical-shaped cross-section. Straight tube sections 358 a and 358 b are elliptically-shaped in cross-section and a U-shaped tube section 356 is also generally elliptically-shaped in cross-section and formed with a concavity 360.

As shown in FIGS. 18-21, a fourth exemplary embodiment of a heat exchanger tube 450 of the present invention has a generally flattened circular cross-sectional U-shaped tube section 356. Straight tube sections 458 a and 458 b are circularly-shaped in cross-section and a U-shaped tube section 356 is generally flattened circularly-shaped in cross-section and formed with a concavity 460.

As shown in FIGS. 22-25, a fifth exemplary embodiment of a heat exchanger tube 550 of the present invention has a generally flattened elliptical cross-sectional U-shaped tube section 556. Straight tube sections 558 a and 558 b are elliptically-shaped in cross-section and the U-shaped tube section 556 is generally flattened elliptically-shaped in cross-section and formed with a concavity 560.

One of ordinary skill in the art would appreciate that the heat exchanger tube of the present invention might be constructed using various combinations of the cross-sectional configurations of the U-shaped tube section and the straight tube sections other than the ones discussed above. By way of example only and not by way of limitation, the straight tube sections might be configured as shown in FIG. 9 while the U-shaped tube section integrally connected to these straight tube sections might be configured as shown in FIG. 25.

A sixth exemplary embodiment of the present invention is a method of forming a straight tube into a U-shaped tube. The method provides a cylindrically-shaped bend die 608 shown in FIGS. 26 and 27. The bend die 608 has a pair of grooves 610 a and 610 b extending into and circumferentially about an outer circumferential surface 612 of the bend die 608. A protuberance 614 is disposed between the pair of grooves 610 a and 610 b and projects outwardly relative to the pair of grooves 610 a and 610 b. Although not by way of limitation, the protuberance 614 is a ring-like member having a rectangular cross-sectional configuration. With the bend die 608, the straight tube is bent about the bend die 608 180° (or within an approximate range of 170° and 190° or whatever bend angle is desired) in a manner to form the U-shaped tube having the U-shaped tube section defining the return bend and the pair of straight tube sections with the U-shaped tube section having the concavity formed thereinto with the concavity defining a portion of the internal space as discussed above.

One of ordinary skill in the art would appreciate that the bend die 608, if split along line S-S shown in FIG. 27, can be incorporated into the tube bending tooling 6 shown in FIGS. 2-6 in order to produce the heat exchanger tubes of the present invention formed with concavities 60. However, a skilled artisan would appreciate that other methods might be used to produce the heat exchanger tube of the present invention formed with concavities 60. For instance, the heat exchanger tube of the present invention can be made using the bending tooling 6 shown in FIGS. 2-6 without using the plastic plug mandrel 14, the plastic follower 16, the mandrel rod 34 and the collet 36. Without these mandrel components and the components associated with the mandrel components, long lengths of straight tubing can be bent to form heat exchanger serpentine tubes having, for example, 12 straight tube sections and 11 return bends with a fluid passage extending, for example, 300 feet.

In FIG. 28, a heat exchanger 710 of the present invention includes an inlet header 712, an inlet connection 714 connected to the inlet header 712, an outlet header 716, an outlet connection 718 connected to the outlet header 716 and a plurality of heat exchanger serpentine tube bodies 80 as described above that interconnect the inlet and outlet headers 712 and 716 respectively. A skilled artisan would appreciate that the heat exchanger 710 is a conventional heat exchanger except that each one of the plurality of serpentine tube bodies 80 includes a concavity 60 as best shown, for example, in FIGS. 7, 8 and 11.

In view of the above, the heat exchanger tube of the present invention with a U-shaped tube section defining a return bend having a concavity formed thereinto results in a reduced pressure loss of the heat exchange fluid as the heat exchange fluid flows therethrough. Further, the heat exchanger tube of the present invention is wrinkle-free even if the heat exchanger tube includes a tight return bend and the heat exchanger tube is fabricated from a material difficult to bend such as stainless steel. Additionally, the serpentine tube of the present invention can be fabricated without the use of a mandrel rod and a mandrel assembly.

The present invention, may, however, be embodied in various different forms and should not be construed as limited to the exemplary embodiments set forth herein; rather, these exemplary embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the present invention to those skilled in the art. For instance, as shown in FIG. 29, the cross-sectional configuration of the U-shape tube section 56 is somewhat distorted from being perfectly symmetrical as illustrated above. Such a distortion might occur when the tube is formed without a mandrel and the mandrel assembly. A skilled artisan would appreciate that such configuration whether perfect or distorted shall be construed as “generally symmetrical”. Further, it is appreciated that all of the objects of the present invention may not be encompassed in each one of the claims. 

1. A tube, comprising: a tube body forming a hollow passageway and having a U-shaped tube section defining a return bend and a pair of straight tube sections with respective ones of the straight tube sections connected to the U-shaped section at respective connection locations and extending generally parallel to one another to define an internal space disposed between and among the U-shaped tube section and the straight tube sections connected to the U-shaped tube section, the U-shaped tube section having a concavity formed thereinto with the concavity defining a portion of the internal space.
 2. A tube according to claim 1, wherein the concavity extends continuously along the U-shaped tube section and terminates adjacent respective ones of the connection locations.
 3. A tube according to claim 2, wherein the concavity has a generally constant depth as the concavity extends continuously along the U-shaped tube section.
 4. A tube according to claim 3, wherein the concavity has a generally tapering depth that lessens commencing from the constant depth as the concavity terminates adjacent the respective connection locations.
 5. A tube according to claim 1, wherein the U-shaped tube section and the pair of straight tube sections are integrally constructed at the respective connection locations.
 6. A tube according to claim 1, wherein respective ones of the straight tube sections as viewed in cross-section have one of circular cross-sections, flattened circular cross-sections, elliptical cross-sections and flattened elliptical cross-sections.
 7. A tube according to claim 1, wherein the U-shaped tube section as viewed in cross-section is generally kidney-shaped.
 8. A tube according to claim 1, wherein the U-shaped tube section includes a main piece connected to the concavity as viewed in cross-section to define a continuous loop, the concavity forming an apex projecting into the hollow passageway and the main piece having a first imaginary reference point disposed diametric to the apex, a second imaginary reference point and a third imaginary reference point and wherein a first axis as viewed in cross-section extends through the apex and the first imaginary reference point dividing the U-shaped tube section generally symmetrically in half-sections with the apex and the first imaginary reference point being disposed apart from one another at a distance therebetween to define a height and a second axis as viewed in cross-section perpendicularly intersecting the first axis extends through the second imaginary reference point disposed on one of the half-sections and the third imaginary reference point disposed on a remaining one of the half-sections, the second and third imaginary reference points being disposed apart from one another at a widest distance therebetween to define a width.
 9. A tube according to claim 8, wherein a ratio of the height to the width is in a range of approximately 0.6 and 0.9.
 10. A tube according to claim 1, wherein at least one of the straight tube sections defines in cross-section a straight tube cross-sectional area of the hollow passageway and the U-shaped tube section defines in cross-section a U-shaped tube cross-sectional area of the hollow passageway, the U-shaped tube cross-sectional area being smaller than the straight tube cross-sectional area.
 11. A tube according to claim 1, wherein the U-shaped tube cross-sectional area is smaller than the straight tube cross-sectional area by an amount in a range of approximately 10% and 30%.
 12. A serpentine tube, comprising: a serpentine tube body having a plurality of straight tube sections and a plurality of U-shaped tube sections, the plurality of straight tube sections arranged in a plurality of generally parallel rows and disposed in a common plane, the plurality of U-shaped tube sections connected to the plurality of straight tube sections in a manner such that a respective one of the U-shaped tube sections defining a return bend connects sequential ones of the plurality of straight tube sections to form a serpentine configuration, each of the sequential ones of straight tube sections connected to a respective one of the U-shaped sections defines an internal space disposed between and among the respective U-shaped tube section and the sequential ones of the straight tube sections, each one of the U-shaped tube sections having a concavity formed thereinto with the concavity defining a portion of the internal space.
 13. A serpentine tube according to claim 12, wherein each U-shaped tube section and sequential ones of the straight tube sections are connected at respective connection locations.
 14. A serpentine tube according to claim 12, wherein the concavity extends continuously along each one of the U-shaped tube sections and terminates adjacent the sequential ones of the straight tube sections at the respective connection locations.
 15. A serpentine tube according to claim 12, wherein each concavity has a generally constant depth as the concavity extends continuously along the U-shaped tube section.
 16. A serpentine tube according to claim 15, wherein each concavity has a generally tapering depth that lessens commencing from the constant depth as the concavity terminates adjacent the respective connection locations.
 17. A serpentine tube according to claim 12, wherein each one of the U-shaped sections as viewed in cross-section is generally kidney-shaped.
 18. A serpentine tube according to claim 12, wherein each one of the straight tube sections defines in cross-section a straight tube cross-sectional area and each one of the U-shaped tubes define in cross-section a U-shaped tube cross-sectional area, respective ones of the U-shaped tube cross-sectional areas being smaller than respective ones of the straight tube cross-sectional areas.
 19. A serpentine tube according to claim 18, wherein each of the U-shaped tube cross-sectional areas is smaller than each of the straight tube cross-sectional areas by an amount in a range of approximately 10% and 30%.
 20. A heat exchanger, comprising: an inlet header; an inlet connection connected to the inlet header; an outlet header; an outlet connection connected to the outlet header; and a plurality of serpentine tube bodies, each serpentine tube body having a plurality of straight tube sections and a plurality of U-shaped tube sections, the plurality of straight tube sections arranged in a plurality of generally parallel rows and disposed in a common plane, the plurality of U-shaped tube sections connected to the plurality of straight tube sections in a manner such that a respective one of the U-shaped tube sections defining a return bend connects sequential ones of the plurality of straight tube sections to form a serpentine configuration, each of the sequential ones of straight tube sections connected to a respective one of the U-shaped sections defines an internal space disposed between and among the respective U-shaped tube section and the sequential ones of the straight tube sections, each one of the U-shaped tube sections having a concavity formed thereinto with the concavity defining a portion of the internal space, the plurality of serpentine tube bodies interconnecting the inlet and outlet headers.
 21. A method of forming a straight tube into a U-shaped tube having a U-shaped section defining a return bend and a pair of straight tube sections with respective ones of the straight tube sections extending generally parallel to one another to define an internal space disposed between and among the U-shaped tube section and the straight tube sections connected to the U-shaped tube section with the U-shaped tube section having a concavity formed thereinto, the concavity defining a portion of the internal space, the method comprising the steps of: providing a cylindrically-shaped bend die having a pair of grooves extending into and circumferentially about an outer circumferential surface of the bend die with a protuberance configured as a ring and disposed between the pair of grooves and projecting outwardly relative to the pair of grooves; and bending the straight tube about the bend die at a selected angle in an approximate range of 170° and 190° in a manner to form the U-shaped tube having the U-shaped tube section defining the return bend and the pair of straight tube sections with the U-shaped tube section having the concavity formed thereinto with the concavity defining a portion of the internal space. 